For nuclear spin tomography, for example, relatively high magnetic fields with additionally relatively high constancy of the respective magnetic field strength over time are required. For this purpose, electromagnets with superconducting coils have been developed. Such coils, which include relatively low temperature (LTc) superconductor material such as niobium-tin or niobium-titanium, have already been known for decades. Such magnets have to be operated in a temperature range of about 4 K temperature.
For about one decade, superconducting materials of the relatively high temperature type (HTc superconductors) have also been known, which are superconductive up to temperatures above that of liquid air, namely at temperatures less than 77° K. Electromagnets with HTc superconducting coils which can be used for relatively high magnetic fields, for example up to temperatures of less than about 40 K, have also already been produced. This lower operating temperature is based on the fact that the HTc current carrying capacity of HTc superconductor materials used for the purpose, for example bismuth cuprates such as (Bi, Pb)2Sr2Ca2Cu3O10 and Bi2Sr2CaCu2O8 and rare earth cuprates RE Ba2Cu3O7, with RE=Nd, Gd, Sm, Er, Y, is only adequate down to a respective operating temperature which is limited as a function of the level of the prevailing magnetic field.
In the ideal case, a short-circuit superconducting current once produced and flowing in such a superconducting coil of a magnet persists. In order to feed such a superconducting current into a superconductor coil, a device known as a flux pump is used, for example. Such a flux pump is disclosed, for example, by “Study of Full-Wave Superconducting Rectifier-Type Flux-Pumps”, in IEEE Transactions on Magnetics, Vol. 32 (1996) pp. 2699-2702 and from “On Fully Superconducting Rectifiers and Flux Pumps”, Cryogenics, May 1991, pages 262-275.
The aforementioned prior art relates exclusively to superconductors of the relatively low temperature (LTc)type, that is to materials such as the aforementioned niobium-tin and niobium-titanium. FIG. 1 shows an example of a flux pump 2 of the rectifier type from the prior art (from IEEE Transactions . . . , as above), in which 11 designates the superconducting coil with LTc superconductor of an electromagnet 111, such as is used for the nuclear spin tomography already mentioned, for example. Numeral 12 designates a current source which supplies the electrical power which is used to build up the superconducting current that flows in the coil 11 during the operation of the electromagnet. Numeral 13 designates a transformer having a primary coil 113 and, in this example, 2 secondary coils 213 and 313 connected in series. Numerals 15 and 16 designate two switches for connecting and interrupting the superconducting current flowing in the circuit of the respective secondary coil 213 and 313, respectively.
In the prior art, these two secondary coils and switches include LTc. In order to be able to act as a transformer 13, the current source 12 designated in general terms supplies an alternating current, that is to say a current with a repeatedly successive opposed flow direction. In accordance with the cycle rate of this change of flow direction, the switches 15 and 16 are opened and closed, specifically in opposition to each other in each case. Rectification of the electric current flowing through the lines designated by 20 and 21 is therefore carried out. This current is the feed current for the coil 11 of the electromagnet. Numeral 23 designates a known safety device, not specifically explained here, to protect the flux pump 2. Numeral 25 designates a control system for controlling the cycle rate of changing the feed current from the current source 12 and the switches 15 and 16.
In the known flux pump of FIG. 1, the switches 15 and 16 are relatively low temperature (LTc) superconductor switches. Their “open” and “closed” states are provided by the “superconducting” or “normally conducting” states of the conductor material contained in them. The superconducting state is present, given an appropriately deeply cooled state. By heating the respective switch element, the latter is converted to the normally conducting state, which corresponds to an opened switch. This conversion is reversible.
In a known way, by periodically switching over the switches 15 and 16, the coil 11 of the electromagnet or its circuit can be charged up gradually with superconducting current, so that correspondingly gradually, a corresponding direct electromagnetic field of high magnetic field strength or high magnetic flux is generated in the coil 11 of the electromagnet, and is permanent if superconduction is maintained. To a wide extent, this permanence applies to the LTc superconduction and the materials used for this purpose and already specified above. For example, a superconductor electromagnet, for example one belonging to a nuclear spin tomograph, once charged up, maintains its magnetic field strength so constantly over a long time that the extremely high requirements on the constancy of the field for nuclear spin tomography can be met with this magnetic field. Recharging is necessary, for example, only after about 100 hours assuming that there are no technical deficiencies or operational errors.
In another connection, specifically for electric stores operating in the cryogenic range with a superconducting winding for extremely quickly available electrical power with a required high peak output, it has been proposed (IEEE, ISPD'99, Toronto May 26-28, 1999, pp. 91-94) to use cool MOSFETs in the electrical control system of such stores, for the switches required there. These MOSFETs are advantageously used at up to 1000 V and at relatively low temperatures around about 77 K, since in this temperature range these MOSFETs have a very low forward electrical resistance. In spite of the relatively high current flow during surge operation, only correspondingly relatively low inherent losses of electrical power are produced in them. In this application, these MOSFETs are therefore used to save power during the operation of such peak output stores.